The present invention relates to a voltage measurement method and apparatus using an electron beam.
With the recent trend of minimization of wiring diameter attributable to high-density and high-integration formation of LSI's, fault diagnosis and defect analysis of LSI's wherein waveform observation and voltage measurement are carried out by using a thin mechanical probe and an oscilloscope have been very difficult to perform. Even in an analysis method which compares and collates the input and output signals of the LSI, it takes a long time to locate and settle a malfunctioning point and frequently, the malfunctioning point is unlocated and unsettled. As an approach to this problem, an electron beam LSI tester has been proposed which uses an electron beam as a probe for voltage measurement (reference literature: "Electron Beam LSI Tester", Hitachi Review 65, 7 (1983, 7) pp. 33-38).
The principle of voltage measurement using an electron beam and a conventional measuring method will now be described with reference to FIGS. 7 to 11.
Referring to FIG. 7, when an electron beam 1 is irradiated on a sample 2, secondary electrons 3 are discharged from the sample 2. Energy of the secondary electrons 3 is analyzed by means of an energy analyzer constructed of two semispherical grids 4 and 5. For extraction of secondary electrons, a constant voltage E4 of +10V to +100V is applied to the inner grid 4. Under this condition, when a voltage E5 applied to the outer retarding grid 5 is changed within a range of, for example, -30V to +30V, the relation between application voltage E5 on the retarding grid 5 and output voltage Es from a secondary electron detector 6 can be displayed in the form of an Es-E5 curve 8 which is similar to the "S" curve 8 shown on a display 7. This curve corresponds to a curve obtained by sequentially integrating an energy distribution of secondary electrons from a higher level of energy. As the sample voltage changes, the position of the S curve shifts.
FIG. 8 illustrates examples of S curves for different sample voltages, indicating that the S curve shifts from 8a to 8b as the sample voltage changes. Since the amount of shift reflects the change in sample voltage, voltage applied to the sample can be measured on the basis of the shift amount of the S curve. More specifically, a slice level 11 is set up in association with the S curves as shown in FIG. 8, retarding grid voltages Va and Vb at intersections of the slice level with the S curves are determined, and a difference (Vb - Va) is measured to determine a change in sample voltage.
If in FIG. 8 the curves 8a and 8b merely shift so as to be saturated at the same level, no problem is raised. However, in the event that the electron beam current changes either during an interval between completion of measurement to determine curve 8a for a known sample voltage portion and commencement of measurement to determine curve 8b, or during the measurement of curve 8a, or in the event that a great amount of contamination is deposited on a portion of the sample on which the electron beam is irradiated, the curve 8b changes to a dashed curve 8b' having a saturation level of S curve which is lower than that of the curve 8b. Accordingly, when the curve 8b' is associated with the aforementioned slice level, there results a large measurement error.
To solve this problem, a measurement method as proposed by, for example, JP-A-61-239554 has hitherto been employed wherein a distribution of normalized secondary electron signal as shown in FIG. 9B is used for measurment. The conventional method will now be described.
FIG. 9A shows an S curve. Secondary electron detector output signals IcO, Icl and Ic2 are determined from the S curve at a certain retarding grid voltage Vg and retarding grid voltages which are .+-..DELTA.Vg shifted from Vg, and the following equation EQU (Ic1-Ic2)/(2.multidot..DELTA.Vg.multidot.Ic0) (1)
is calculated. By performing this calculation over the entire range of the retarding grid voltage, the normalized secondary electron signal distribution of FIG. 9B can be obtained. This distribution is advantageous in that the change in secondary electron signal due to the change in electron beam current or the deposition of contamination is cancelled out, and therefore it is preferable that the sample voltage be determined on the basis of a shifting of the distribution. More specifically, as in the case of the S curve described with reference to FIG. 8, a slice level is set up and a difference between retarding grid voltages is measured to determine a change in sample voltage.
When an AC voltage such as a rectangular wave is applied to the sample, a waveform and a voltage at a desired time point within the waveform can be measured according to a method as will be described below with reference to FIG. 10.
In FIG. 10, a rectangular wave is supplied to a sample 2. A constant voltage E5 is supplied to a retarding grid 5. Secondary electrons 3 generated from the sample 2 under irradiation of an electron beam pass through the retarding grid 5 and are detected by a secondary electron detector 6. The secondary electron signal produced from the detector 6 changes as the sample voltage changes. The changing secondary electron signal is converted into a digital signal at the rate of, for example, 5 nanoseconds (10.sup.-9) by means of a high-speed A/D converter 10 succeeding the secondary electron detector 6 and fetched into a signal processor 11 comprising a memory and the like components. The output signal of the detector is displayed with respect to time lapse on a display 7 to provide an AC waveform which can be observed. In order to determine an absolute value of such an AC waveform, the voltage E5 to be applied to the retarding grid 5 is changed as in the case of DC voltage and the AC waveform is fetched each time the voltage E5 is changed so that S curves may be acquired at all time points, and thereafter normalized secondary electron signal distributions are prepared and a shift amount of distribution is measured to determine a sample voltage.
When the AC waveform supplied to the sample is of high speed and high frequency, AC waveform measurement based on a stroboscopic method is carried out.
FIG. 11 illustrates a basic arrangement for AC waveform measurement based on the stroboscopic method. A pulse generator 21 is a power source used to pulse an electron beam 1 and it is triggered and driven by a delay circuit 22. The delay circuit 22 is adapted to sequentially delay timings for irradiation of the pulsed electron beam 1 on a sample 2 and controlled by a computer 15. With this construction, when the pulsed electron beam is irradiated on the sample with the delay circuit 22 fixed at a certain timing (phase), secondary electrons discharged at that timing are detected. Subsequently, the timing (phase) of the delay circuit is shifted slightly and secondary electrons are detected in a similar manner. By repeating the above operation by, for example, 1024 times over one period of the waveform, even the high-speed and high-frequency waveform can be measured with ease. In this case, voltage E5 to be applied to the retarding grid is changed at the individual timings to prepare normalized secondary electron signal distributions and the waveform is displayed in the absolute value form.
The method of obtaining the absolute value by preparing the normalized secondary electron signal distribution faces no problem when the object to be measured is DC voltage.
However, when the method is applied to the AC periodic voltage to be measured by using the aforementioned high-speed A/D converter and stroboscopic method, S curves must be measured at a plurality of points constituting a waveform and disadvantageously the measurement time is increased. Especially, the method can not be used practically to measure such a sample as a logic LSI having a very long operation period.
Therefore, a voltage measurement apparatus based on the above measuring method can not determine and display, with ease, an absolute value of voltage at a desired time point within an AC waveform to be measured and absolute values over the entire AC period.